A liquid crystal display device has a liquid crystal panel unit and a backlight unit that is arranged on the rear surface of the liquid crystal panel unit. The liquid crystal panel unit adjusts the transmittance of light (the amount transmitted) from the backlight unit to display images on the front surface of the liquid crystal panel unit.
These backlight units are largely divided into two types. One type is a light guide plate type (an edge-lit type) in which light enters from a side face of the light guide plate, and the other type is a direct-lit type in which light sources are arranged on the rear surface of the liquid crystal module.
Conventionally, an edge-lit backlight unit had a structure in which light was incident on a side face of the light guide plate, which made it difficult to emit a large planar light with a uniform brightness distribution. This edge-lit backlight unit was frequently used in small liquid crystal display devices, such as for the monitors of notebook PCs and gaming systems. Due to the recent rise in demand for liquid crystal display devices to be thinner and smaller along with improvement in light guide plate precision, and because of an increase in brightness of the LEDs used as light sources and the like, it has become possible to emit large planar light with a uniform brightness distribution, and thus, the edge-lit backlight unit has been increasingly used in large liquid crystal display devices, such as in large-sized televisions.
The edge-lit backlight unit will be explained below. The edge-lit backlight unit includes: a light source unit in which a plurality of LEDs are arranged in parallel; a light guide plate that receives light emitted from the light source unit on a light receiving face on a side face of the light guide plate, the light guide plate emitting planar light from a light exiting surface that is one main surface of the light guide plate; optical sheets arranged adjacent to the light exiting surface of the light guide plate; and a reflective sheet arranged adjacent to the surface of the light guide plate that is opposite to the light exiting surface. These members are arranged inside a backlight chassis.
In such an edge-lit backlight unit, it is preferable for the light source unit to be as close to the light guide plate as possible in order to reduce unevenness in light emitted from the light source unit. The light guide plate and light source unit, however, are arranged with a gap therebetween to prevent the light guide plate and light source unit from coming into contact with each other when the light guide plate expands due to heat.
Meanwhile, the light emitted from the LEDs is diffused light, and this light emitted from the LEDs may leak from the gap, without entering the light guide plate, and then be irregularly reflected by the optical sheets, reflective sheet, and the like, thereby being emitted to outside of the backlight unit (light leakage). If this light leakage occurs, a line with a high brightness (hereinafter, a bright line) will occur in the vicinity of the light source unit that emits the planar light from the backlight unit. If this bright line occurs, the uniformity of brightness of the planar light will be lost, and the display quality of images displayed by the liquid crystal display device will be reduced.
This light leakage often occurs due to the size of the LEDs, the thickness of the light guide plate, and the gap between the light source unit and the light guide plate. Recent backlight units have a tendency for the light guide plate to be smaller (thinner) in order to be lighter and cheaper, but it is difficult to make the LEDs smaller to the same degree as the light guide plate because the light emitting efficiency of the LEDs would be reduced. As such, if the light guide plate becomes thinner relative to the size of the LEDs, then the diffused light from the LEDs is more susceptible to straying from the light receiving face of the light guide plate and leaking, even if the gap between the light source unit and the light guide plate remains the same.
In Japanese Patent Application Laid-Open Publication No. 2007-128748, a backlight configuration is proposed in which a reflective plate is arranged above the gap between the light source unit and light guide plate, and a reflective sheet with diffuse reflection characteristics is bonded to a surface of the reflective plate near the light guide plate. In this backlight, light that is emitted by the LEDs but not incident on the light guide plate is reflected by the reflective sheet bonded to the reflective plate and then incident on the light guide plate. This suppresses the occurrence of light leakage and uneven brightness in the planar light caused by the light leakage.
In Japanese Patent Application Laid-Open Publication No. 2008-71696, a backlight is proposed in which the light emitting part of the LEDs is arranged at a slant and sealed with a resin in order for light to enter a thin light guide plate. By providing a reflective member that reflects light emitted from the sealing resin, the light reflected by the reflective member enters the light guide plate. With this configuration, the outer diameter of the LEDs can be made smaller, and the backlight can be made thinner.
In Japanese Patent Application Laid-Open Publication No. 2006-222430, a configuration is proposed in which a beam shutter is formed that shields regions of an LED sub-mount or circuit substrate where light leakage is undesirable from being irradiated with light. A configuration is also disclosed in which this beam shutter has reflective characteristics. With this configuration, light emitted from the LEDs is blocked by the beam shutter, and thus, light leakage of the backlight can be suppressed.